Protection of magnesium-rich alloys



Patented Dec. 10, 1940 PATENT OFFICE 2,224,245 PROTECTION or MAGNE SIUM-RICH ALLOYS Frank Arthur Allen, Windsor, England, assignor to High Duty Alloys limited, Slough, England No Drawing. Application February 7, 1939, Se-

rial No. 255,165.

9 Claims.

This invention relates to the protection against corrosion and erosion of articles composed of or including portions composed of magnesium rich alloys.

The employment of magnesium rich alloys has heretofore been considerably restricted owing to their low resistance to corrosion by sea water and other causes and to erosion, the resistance varying inversely with the magnesium content and being least in magnesimn base alloys in which magnesium predominates, and the present invention has for its object to remove this disadvantage.

In accordance with the present invention, the process for the treatment of articles composed of, or including portions composed of, magnesium rich alloys consists in immersing the article or the relevant portion thereof in a heated aqueous solution "of alkali dichromate and nickel sulphate, chromium sulphate or nickel ammonium sulphate to which has been added an organic acidifying.

agent producing in such aqueous solution or maintaining it at a pH value between 2.0 and 6.4

w and which value is advantageously not less than solution is adjusted to 3.8 as is that of the organic acidifying addition which may conveniently be made up of acetic acid and sodium or ammonium acetate.

In the practice of the invention according to one mode of carrying out the process, 50 gallons of solution is made up of the following composition:

1.5% Chromium sulphate lbs 7 3.0% Potassium dichromate lbs 15 0.625% Acetic acid pints 2 0.8% Sodium acetate lbs.. 4 Water to make '50 gallons.

Die castings acid dipped in 5% sulphuric acid and immersed in this solution. at boiling point chromate perfectly, without difficulty, in 20 minutes, there being no necessity for any acid addition and the reaction being gentle with the t In Great Britain September semblance that the deposit darkens from dark brown to black on drying. The treated castings Another aspect of the present invention, consists in the preliminary treatment of stampings, sand castings or dirty articles including or com posed of magnesium rich alloys with a view to ensuring a satisfactory deposit on immersion.

This pretreatment is effected in a bath, preferably cold, consisting of 5-20% chromic acid and 240% nitric acid in water, which is found effectively to remove partly carbonized grease and embedded dirt in stampings and the like in ten minutes and when followed by a quick sulphuric acid dip, gives perfectly bright articles. The preferred composition is 8% chromic acid and 5% nitric acid. It is noteworthy that the action of this chromic/nitric acid bath is quite gentle and much less vigorous than that of either of the component acids separately.

By the present invention improved processes for the protection of articles composed of, or including portions composed of, magnesium rich alloys are obtained.

What I claim is:

1. A process for the treatment of articles composed of magnesium rich alloys consisting in im mersing the article in an aqueous solution of alkali dichromate and metallic sulphate to which has been added an organic acidifying agent main. taining such aqueous solution at a pH value between 2.0 and 6.4.

2. A process for the treatment of articles composed of magnesium rich, alloys consisting in immersing the article in a heated aqueous solution of alkali dichromate and metallic sulphate to which aqueous solution has been added an organic acidifying agent rendering and maintaining it at a pI-l value between 3.4 and 4.3.

3. A process according to claim 2, wherein the acidifying agents consist of organic acids and salts which only slightly ionise in solution and therefore successively liberate further ions as the process continues.

4. A process for the treatment of articles composed of magnesium rich alloys consisting in immersing the article in a heated aqueous solution oi alkali dichromate and metallic sulphate, to which has been added an organic acidifying agent, the pH value of such aqueous solution and of the organic addition being each adjusted to 3.8.

5. A process according to claim 2 wherein the acidifying addition consists of acetic acid and sodium acetate.

6. A process according to claim 2 wherein the organic acidifying addition consists of acetic acid and ammonium acetate.

7. A process according to claim 2 wherein the aqueous solution contains the following ingredients per 50 gallons:

1.5% Chromium sulphate lbs 7 /2 3.0% Potassium dichromate ..1bs 15 0.625% Acetic acid "pints-.." 2

0.8% Sodium acetate lbs 4 8. A process according to claim 2 when applied to stampings, sand castings and dirty articles including the preliminary step of immersing the articles in a bath consisting of 8% chromic acid and 5% nitric acid in water, whereby grease and embedded dirt is removed.

9. A process according to claim 2 when applied to stampings, sand castings and dirty articles including the preliminary stepof immersing the articles in a bath consisting of 8% chromic acid and 5% nitric acid in water, whereby grease and embedded dirt is removed, such pretreatment including a quick sulphuric acid dip.

FRANK ARTHUR ALLEN. 

